Lymph node metastasis is an important prognostic factor in gastrointestinal cancers, such as gastric cancer and colon cancer. Accurate diagnosis of the presence of lymph node metastasis is essential to determine appropriate treatment for patients.
For example, the necessity of postoperative adjuvant therapy for many gastrointestinal cancers is determined by the presence of lymph node metastasis (NPL 1 and NPL 2). In particular, when the diagnosis of gastric cancer with intraoperative sentinel lymph node biopsy shows that there is no metastasis, surgery with reduced lymph node dissection may be selected (NPL 3). For these reasons, rapid and accurate diagnosis of lymph node metastasis would be clinically very useful.
However, no diagnostic technique has been established at present to replace conventional histopathological diagnosis. Routine histopathological diagnosis of lymph node metastasis is performed only on samples obtained from only a single or a few sections; therefore, it is possible to miss some cases of micrometastasis, and its diagnostic accuracy is not sufficient (NPL 4). Moreover, intraoperative rapid diagnosis takes at least 20 to 30 minutes before the diagnosis. It is thus necessary to establish a new, more accurate and rapid approach.
Photodynamic technology using 5-aminolevulinic acid (5-ALA) is currently applied for cancer detection in a wide region including the gastrointestinal region (NPL 5, NPL 6, and NPL 7). 5-ALA is a kind of endogenous amino acid, and when exogenous 5-ALA is administered, protoporphyrin IX (PpIX), which is a metabolite of 5-ALA, accumulates in cancer cells due to the difference in the activity of metabolic enzymes. The principle of this technology is to detect PpIX, which is a fluorescent material, for cancer diagnosis. The present inventors' group reported very excellent results of the diagnosis of lymph node metastasis in a murine model of rectal cancer by a method using 5-ALA-induced PpIX fluorescence (NPL 4). This was the first report on photodynamic diagnosis using 5-ALA for lymph node metastasis of gastrointestinal cancer.
Photodynamic technology using 5-aminolevulinic acid (5-ALA) is currently applied for parathyroid gland detection (NPL 10).
However, lymph nodes in the human body are wrapped in connective tissues. In other words, lymph nodes in the human body contains rich connective tissues. Connective tissues (e.g., fat, collagen, etc.) emit strong autofluorescence in the blue to green wavelength range upon irradiation with blue excitation light. This autofluorescence interferes with the detection of PpIX fluorescence.
It is known that PpIX is photobleached by light irradiation, and that a photooxidation reaction occurs during photobleaching to convert PpIX into another substance called photo-protoporphyrin (PPp) (NPL 8). PpIX has a fluorescence peak at 635 nm, while PPp has another fluorescence peak at about 675 nm (NPL 9).